This is a chart I took from Bruce Clay, Inc. that shows where each major search engine gets their results from. By looking at this you can decide which search engines to put most your efforts in.

I just love charts. It simplifies everything.
Here is what Bruce Clay believes is the “SEO Hierarchy of Needs“.
This helps you prioritize your SEO campaign. I agree with the elements of this pyramid. The spaces between each level are very close and you need to concentrate on all of these things in order to run a successful website. So i can’t say that one is more important than the other, but only that you need the base before you begin to move up the pyramid.